The torsional potential for phosphate diesters. The effect of geometry optimization in CNDO and ab initio molecular orbital calculations

CNDO and ab initio calculations on the dimethyl phosphate monoanion demonstrate that geometry optimization can dramatically alter the relative energies of the conformational isomers of the phosphate ester. Thus, with geometry optimization (particularly of the diester RO-P-OR bond angle) the energy differences between the gauche-gauche, gauche-trans, and trans-trans conformations is less than 1 kcal/mol rather than approximately 7 kcal/mol previously reported. A torsional energy map for the two ester torsional angles which includes ester bond angle optimization is presented. Ab initio (STO-3G level) and CNDO calculations confirm the strong coupling of the diester RO-P-OR bond angle to the torsional conformation.